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Astronomers find stronger evidence of supermassive black holes

January 13, 1997

TORONTO (AP) _ Supermassive black holes, once known only in theory, may be sucking in gas and stellar matter at a voracious rate in the center of virtually every large galaxy in the universe.

That’s the conclusion of astronomers who say new observations by the Hubble Space Telescope have detected the ``celestial fingerprint″ of three black holes, including one that is a half-billion times more massive than the sun. A survey of 15 other galaxies suggests that 14 of them also have black holes.

The discoveries are the strongest observational evidence yet for black holes, an exotic and elusive object first predicted by Albert Einstein in his theory of general relativity.

``The mystery hasn’t been solved yet, but we are convinced we have found the body,″ said Scott D. Tremane of the Canadian Institute for Theoretical Astrophysics and a member of a team of black-hole hunters.

``Einstein’s relativity theory is correct,″ said one astronomy slide. ``Cosmic vacuum cleaners are real.″

The black-hole studies were presented Monday at a meeting of the American Astronomical Society.

Perhaps the most violent and powerful points of energy in the universe, black holes form when matter gathers at a single place, often from a collapsing, massive star. This point, drawing in more and more matter, becomes so dense that nothing can escape, not even light. It becomes black and unseen. Anything caught up in its center is removed forever from the rest of the universe.

``Black holes,″ said Ralph Narayan of the Harvard-Smithsonian Center for Astrophysics, ``are the ultimate victory of gravity.″

To find evidence of the galactic black holes, the astronomers analyzed the motion of stars and superheated gas near the center of galaxy clusters 50 million light years from Earth. The extreme velocity of the stars showed they were being towed by a powerful gravity force. Gas, accelerated at near light speed, is heated to a trillion degrees.

From these observations, the astronomers could calculate the mass of the object sucking in its neighbors. One black hole, in the constellation Virgo, was estimated at 500 million times more massive than the sun. Another was 100 million times the sun and the third was 50 million.

``There is something there, it weighs a lot, it is completely dark and we can’t think of anything else it could be,″ said Tremane.

The force of a black hole’s gravity is so great that whole stars nearby are pulled into a spiral motion. Gas whirls about the center at such speeds that it sears to temperatures unknown anywhere else and bursts bright with energy. A black hole cannot be seen, but its effects on nearby stars and gas clouds are highly visible.

An international team of astronomers that conducted the black-hole search said the findings suggest these results:

_Supermassive black holes are common, dwelling in the center of nearly every large galaxy, including the Milky Way.

_The mass of a black hole is proportional to the host galaxy. The larger the galaxy, the more massive black hole.

Doug Richstone of the University of Michigan, leader of the team, said the galactic black holes may be ``fossil quasars,″ or the remnants of a special type of star that collapsed after 100 million years or so.

Quasars, now rare, once were the most luminous and energetic objects in the heavens. Those that have been studied are the most distant and oldest objects in the universe, said Tremane.

``They are so powerful that it defied our thought to find a way to fuel them,″ said Tremane. ``The only viable source would be an infall of material into a supermassive black hole.″

Quasars, he said, have a limited lifetime, ``but black holes go one forever.″

Though the black holes are voracious in their gravitational appetite, Tremane said, they will never swallow whole galaxies that have millions of stars.

``It is a very inefficient cosmic vacuum cleaner,″ he said. The throat of the black hole is so small and galaxies so huge, that only about 1 percent of the galactic matter will ever be consumed, said Tremane.

While Richstone and his group studied supermassive black holes dwelling at the center of galaxies, Narayan and his group at Harvard-Smithsonian studied less powerful stellar black holes, thought to form from the collapse of smaller stars.

Narayan said his group studied X-ray novae, which are companion stars that put out X-ray signals. They concluded that gas is being pulled from one star to its companion at such a speed that it heats to more than a trillion degrees and spews X-rays.

If the receiving star is a black hole, Narayan said, only about 1 percent of the energy is radiated. The rest disappears across an ``event horizon,″ a gravitational sphere that is a ``point of no return″ surrounding the black hole.

``The region inside the event horizon is completely cut off from the outside world forever,″ said Narayan.

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